A common practice used today to inspect the accuracy of position, size, orientation and shape of a plurality of holes, such as, equally spaced irregular shaped slots disposed in a piece part utilizes a manual mechanical comparative method. An example, of such a piece part includes a ring or shroud of a nozzle or stator of a gas turbine engine. The irregular shaped slots provide pilot holes for vanes of the stator or nozzle which are affixed by brazing. The manual inspection method requires that the slotted ring is optically magnified many times by a series of lenses, mirrors, and the like. The final magnification of many times size is overlaid by a transparent film upon which an outline of the target (nominal) irregular shaped slot is drawn. When the outline of the magnified actual slot lies within a preselected tolerance range of the outline of the target slot, the slot is determined by the examining person to be acceptable. Each slot of the slotted ring of slots is measured in this manner.
This manual technique has been used only when absolutely necessary because it takes a substantial amount of time to accurately position the piece part and accurately position the overlay relative to each magnified slot being inspected. It should be recognized that this technique also requires a significant amount of skill on the part of the person inspecting the piece part. As a result, the cost of each piece part is substantially increased. Further, such manual methods require a substantial amount of judgement on the part of the inspector which may result in unacceptable variations in tolerances between piece parts. These variations in tolerance can cause marginally acceptable piece parts to be scrapped or marginally unacceptable piece parts to be accepted. A marginally acceptable stator or nozzle can adversely affect the performance of the gas turbine engine in which it is installed. It is therefore evident that a method and apparatus for consistently inspecting a plurality of elongate slots disposed in a ring is required in order to improve the quality of inspection of each piece part. Such a method and apparatus would facilitate more frequent inspection of piece parts since the time to perform such an inspection would be substantially less.
One problem associated with inspecting an irregular shaped slot relates to the fact that the slot has no easily defined key characteristics or unique features. For example, an irregular shaped elongate slot lacks a true center, distinct opposite ends, parallel sides and the like. Therefore, key elements that could be significant in determining the accuracy of the irregular elongate slot, such as the geometric center, area, and angle of orientation are not readily determined. Therefore, an apparatus or method to automate or accurately the measure these parameters has not been pursued.
For those involved in punching irregular shaped slots in stator rings or shrouds, it would be beneficial to be able perform the inspection of the slots of the punched rings shortly after the punching process has been completed, in order to eliminate the step of accurately positioning the ring. This, however, seems to be unlikely as the lubricant used in the punching process would interfere with the use of any visual inspection device. A visual inspection device would become contaminated with lubricant dirt and the like. Further, in an environment such as this, lighting of the piece part for inspection purposes is inadequate. To provide adequate lighting in such an environment is plagued with problems and would cause one to abandon any thought of associating the inspection of the piece part with the punching apparatus.
The invention is directed to overcoming one or more of the problems as set forth above.